The present invention relates to an electrophotographic image forming apparatus including a plurality of image forming stations.
An electrophotographic image forming process includes a transfer process in which a developed image formed with developer (hereinafter referred to as a developer image) on a surface of an image carrier is transferred to a sheet material transported near the image carrier.
In the transfer process, a transfer bias voltage is generally applied to a transfer electrode which is in contact with a surface of the sheet material, the surface being a reverse side with no image to be formed thereon. Typical examples of the transfer electrode are a transfer roller and a transfer/transport belt. The transfer bias voltage is typically from about +1.5 kV to +4 kV of opposite polarity to the developer.
The transfer bias voltage is usually opposite in polarity to an electric potential of the image carrier. There is thus possibility of the surface of the image carrier being irregularly charged by contact with the transfer electrode where the transfer bias voltage is applied. The irregular charging of the image carrier occurs more likely when there is no sheet material between the transfer electrode and the image carrier and thus the transfer electrode are in direct contact with the image carrier.
This is why some conventional image forming apparatuses have utilized a control method by which a transfer bias voltage is applied to a transfer electrode in a timely manner when a sheet material is transported in between an image carrier and the transfer electrode.
However, it is hard to apply the transfer bias voltage to the transfer electrode in accurate timing with transportation of a sheet material. A slight delay in applying the transfer bias voltage causes the fact that a developer image is not transferred to a front portion of a sheet material. If the transfer bias voltage is applied untimely, the image carrier is charged irregularly, causing image quality deterioration.
In the aforementioned method, moreover, the transfer bias voltage is not applied to the transfer electrode when there is no sheet material between the image carrier and the transfer electrode. Accordingly, the transfer electrode can be negatively charged under the influence of the image carrier having a negative charge. This results in the problem that the transfer electrode cannot be charged appropriately even though the transfer bias voltage is applied thereto.
Thus, in some conventional art methods, when there is no sheet material between an image carrier and a transfer electrode, a voltage of the same polarity as the transfer bias voltage is applied to the transfer electrode, the voltage being very low compared to the transfer bias voltage. The conventional art argues that it can prevent the transfer electrode from being negatively charged while the transfer process is not performed, thereby allowing appropriate application of the transfer bias voltage in the transfer process. The conventional art also argues that it can prevent irregular charging of the image carrier and the transfer electrode even when they are in direct contact with each other.
The foregoing method for controlling the transfer bias voltage is disclosed in JP H02-39181 A (see line 11, upper right column on page 3, to line 8, upper left column on page 4, and FIG. 3), JP H05-150577 A (see paragraphs [0021] to [0023], FIGS. 1 and 2), and JP H10-142893 A (see paragraphs [0044] to [0047], FIG. 1), for example. A power unit for controlling the transfer bias voltage as described above is disclosed in JP H07-181814 A and JP H07-20727 A, for example.
However, in a tandem-type image forming apparatus provided with a plurality of image forming stations, the aforementioned method for controlling the transfer bias voltage sometimes cannot be utilized appropriately. As a typical example of the tandem-type image forming apparatus, let us consider here an image forming apparatus for forming a multi-color image on a sheet material transported by a transfer electrode provided in endless form, by sequentialy transferring onto the sheet material developer images formed in the respective image forming stations.
In this tandem-type image forming apparatus, if the aforementioned method for controlling the transfer bias voltage is used in the image forming stations, the voltage is sequentially applied to the transfer electrode when the transfer process is not performed. Thus, a positive potential of the transfer electrode increases to a higher level than necessary. This causes excessive amount of developer to be transferred to the transfer electrode, resulting in problems such as image quality deterioration and developer wastage. It also causes irregular charging of the image carrier that is in contact with the transfer electrode.